Apr 20 2011 | Posted by SSSandy

New shapes in granular movement

University of Sydney researchers have discovered new patterns in the movement of granular mixtures with findings that could assist the study of oil and gas recovery and CO2 geo-sequestratrion.

Results obtained by lead researcher Dr Bjornar Sandnes, published in Nature Communications today, show the answer to the question 'What happens when you inject air into a mixture of sand and water' yields surprising and often beautiful imagery.

Dr Sandnes performed numerous experiments whereby tiny sand-like particles sandwiched between two plates were injected with air, forcing movement. He altered rates of air injection, air quantities and density of material in granular mixtures to produce previously unobserved patterns.

Mixtures with low granular density and subjected to low to medium rates of air injection – frictional fingering, stick slip bubbles and fluidised front in the above diagram – yielded new patterns.

“The interaction of single particles when they get together was pivotal to this research,” Dr Sandnes says. “If you push one particle, the impact spreads through networks of particles, or 'force chains', within the material. We then see how simple interactions, when repeated, can lead to complexity on a larger scale.

Dr Sandnes says his paper Patterns and flow in frictional fluid dynamics has consequences for altering material flows. Interactions can, for instance, lead to increased friction causing material to “jam up”. When this happens, air pressure must be ramped up to restart flows, resulting in repeated expansions of large bubbles in the mixture. Grains packed closely require high pressure in order to enable penetration by air, causing the air to rip through material in sudden fractures. Other experiments give “frictional fingers” and “corals” and a range of other displacement structures.

“Replacing the simple ingredients used in his research offers a recipe for studying a range of processes relevant for oil and gas recovery, CO2 geo-sequestration, and industrial processing of pastes, slurries, and granular suspensions,” he says.

“While our job is to develop an understanding of the physics, and the implications for industrial processing, you can't help but marvel at the beauty. We provide the canvas then nature makes the art.”

Dr Sandnes is a visiting fellow at the Faculty of Engineering and Information Technologies. Patterns and flow in frictional fluid dynamics was co-written by Dr Sandnes' colleagues at the University of Sydney and the University of Oslo.